A virtual machine (VM) is a software implementation of a machine (for example, a computer) that executes programs like a physical machine. System virtual machines provide a complete substitute for the targeted real machine and a level of functionality required for the execution of a complete operating system. A virtual machine manager known as a hypervisor uses native execution to share and manage hardware, which allows multiple different guest operating systems, isolated from each other, to be executed on the same physical machine. Modern hypervisors use hardware-assisted virtualization, which provides efficient and full virtualization by using virtualization-specific hardware capabilities, primarily from the host CPUs.
A virtual hard disk, or simply a virtual disk, is a disk image file or files that appear as a physical disk drive to a guest operating system. These vdisks store the contents of the virtual machine's hard disk drive and can be stored anywhere the physical host can access. Virtual disks come in various formats, which are typically specific to a vendor or hypervisor, and new incompatible formats are constantly created.
Enterprises today have complex environments with hypervisors from multiple vendors, and reliable migration of virtual machines across hypervisor platforms is a complicated and time-consuming task.